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Superconducting cavity developments for the next generation of ISOL facilities (HIE ISOLDE)

Superconducting cavity developments for the next generation of ISOL facilities (HIE ISOLDE). Walter Venturini Delsolaro (CERN BE-RF-SRF). ENSAR/NA3 Meeting, 28/29 May 2013. Acknowledgement.

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Superconducting cavity developments for the next generation of ISOL facilities (HIE ISOLDE)

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  1. Superconducting cavity developments for the next generation of ISOL facilities (HIE ISOLDE) Walter Venturini Delsolaro (CERN BE-RF-SRF) ENSAR/NA3 Meeting, 28/29 May 2013

  2. Acknowledgement This presentation is based on the work of many people at CERN and outside. It would be very difficult to acknowledge all contributions singularly without forgetting anybody. Particular thanks to S. Calatroni and the CERN coating team; to G. Bisoffi, E. Palmieri, A. Porcellato, S. Stark and the INFN-LNL staff, and to S. Bousson and the IPN cavity testing team in Orsay

  3. Overview • Technologies for superconducting quarter wave resonators • Nb sputter cavities • Nb-Cu sputtered QWR at LNL-INFN for the ALPI Linac • The HIE ISOLDE project, Nb-Cu sputtered QWR at CERN • Latest results of HIE ISOLDE cavities

  4. Technologies for SC QWR • Bulk Nb with high RRR and EB welds • Available from industry: • High gradients at low dissipated power are easier • Difficulties in operation (microphonics, high RF power needs) • Nb clad- copper • External conductor in N-Cu, shaft in bulk Nb • High performances (sensitive to Q disease) • Superconducting coatings (mechanical and thermal stability, lower cost) • Electroplating of Pb on Copper • Limited to few MV/m due to low Bc of lead • Might still be interesting for complicated shapes • Nb sputtering on copper Higher performance than lead plating, can compete with bulk Nb in the 100 MHz and at 4.2 K

  5. Nb sputter technology (history) • It all started at CERN in the early 80’s (C. Benvenuti, N. Circelli, M. Hauer, Applied Physics Letters 45, 583 ,1984) • The magnetron sputtering technology was chosen for the phase 2 of LEP and industrialized • 268 Nb/Cu elliptical cavities (352 MHz) installed in LEP • 16 Nb/Cu elliptical cavities (400 MHz) installed in LHC • CERN continued the research on elliptical cavities during the 1990 (see for example C. Benvenuti, S. Calatroni, M. Hakovirta, H. Neupert, M. Prada and A.-M. Valente, Proceedings of the 10th workshop on RF Superconductivity, 2001, Tsukuba, Japan)

  6. Nb sputter on Cu technology The good features The drawback • Thermally stable (initial motivation) • Much cheaper raw material • Possibility to re use the same substrate, no scrap material, replace bad coatings • Stiffness, no microphonics, can work with narrow BW if no beam loading • Power coupler simplified, no high power RF, no active tuning and complicated feedback systems • Less sensitive to earth magnetic field: saving on magnetic shielding • Possibility for new SC materials • For high frequency, high field (2 K) applications; outperformed by bulk Nb • Higher residual surface resistance • Q slope • Much less industrialized

  7. More history: the Nb-Cu QWR for ALPI in INFN-LNL • Research program was started in 1988 ( V. Palmieri, R. Preciso, V. L. Ruzinov, S. Yu. Stark, L. Badan, A. M. Porcellato, Proceedings of the 5th Workshop on RF superconductivity, 1991) • By 1993 reliable results were reached: three prototypes overcome 6 MV/m at 7 W • 1995 first 4 resonators installed in ALPI (performance degradation on line) • 1997 new series of QWR with improved design : 5.7- 7 MV/m at 7 W • 1998 second cryostat with 4 QWR operates at 6 MV/m with beam, R/D is stopped • 1999 ALPI upgrading program launched: turn old Pb plated to Nb sputtered cavities • 2003 the whole medium beta section of ALPI is upgraded  +60% energy gain Evolution of resonator geometry (from V. Palmieri, V. L. Ruzinov, S. Yu. Stark, L. Badan, A. M. Porcellato, R. Preciso, F. Chiurlotto, M. Morvillo; Proceedings of the 6th Workshop on RF superconductivity, 1993)

  8. ALPI cryostat Bias sputtering configuration From Pramana- Journal of physics, Vol 59, No. 5, November 2002, pp. 871-880 From www.lnl.infn.it

  9. HIE ISOLDE cavities A. D’Elia, R. M. Johnes, M. Pasini, Proceedings of SRF2009, Berlin, Germany

  10. Sputtering system G. Lanza, S. Calatroni, L. Marques Antunes Ferreira, A. Gustafsson, M. Pasini, P. Trilhe, Vincenzo Palmieri; Proceedings of SRF2009, Berlin, Germany

  11. CERN Sputtering system for QWR

  12. Nb sputteredcavity

  13. RF tests in vertical cryostats In 2012, 10 test cavities (with parameters progressively closer to the ALPI sputtering protocol), qualified at 4.5 K New version of fundamental power coupler qualified Dedicated experiments done to assess the contribution of the bottom plate contact to the total loss Dedicated experiment done to assess the sensitivity of the cavity Q to stray magnetic field from the superconducting solenoid Dedicated experiment done to check the possibility that Q switches originated in the transition of the bottom plate to the normal conducting state Improvements to the test setup (infra red lamps, mobile coupler, spare inserts, logistics)  Turnaround of 2 weeks demonstrated

  14. Q2_5: November 2011first cavity with bias diode method and increased coating temperature For the first time Eacc of 6 MV/m was reached Q still too low, an order of magnitude Possible ohmic source (bottom plate?) Measurement with In gasket excluded major effect

  15. Q1_10: March 2012SS support (reduced temperature gradient during coating); helicoflex gasket (improved vacuum) The positive trend on Q0 with increasing dN/dt and T continued, but Q slope also increased: no much gain at 6 MV/m

  16. Q1_11: July 2012Several changes done to approach the ALPI parameters: IR heaters with copper screens, temperatures, power, sputtering gas, venting gas “Quantum jump” in Q0 Bad film quality on the tip of the inner conductor Field emission at very low field

  17. Q switches

  18. Q2_7: October 2012 Increasing film thickness by 25% with same parameters as Q1_11 Reached 5 MV/m at 10 W (5.3 MeV/a for A/q of 4.5!*) No Q switches! Then degraded and became limited by field emission *With 10 cavities in Phase 1

  19. 2013 developments • At the end of 2012 the main remaining issues wereclearly identified • Coating rate (thickness) on cavity top was too low • Poor surface quality on tip of inner conductor (peak E field region) • Q switches • Test in Orsay of a cavity affected by Q switch confirmed results at low field, but without Q switch • Q switch proved to be an extrinsic effect likely due to tuning plate • The cathode distance to the cavity top was decreased by 20 mm on a real cavity (Q3.4) The same configuration was reproduced on a sample run (Q4.3) •  Coating rate increased by a factor two on the cavity top The Q3.4 cavity reached 6 MV/m at 10 W dissipated power, the HIE ISOLDE Specification

  20. First test in Orsay (February 2013)

  21. Second test in Orsay (April 2013)

  22. Summary • The Nb sputter on Cu technology for SC cavities, invented at CERN and used for LEP and LHC, is particularly interesting for RIB facilities • INFN-LNL developed it for QWRs and used it to upgrade the ALPI heavy ion linac • The same technology was chosen for HIE ISOLDE • Development phase started at CERN in 2008, end 2012 cavity performance reached satisfactory levels • Recently HIE ISOLDE specs were achieved on a prototype cavity

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